Cardiac pacemakers are medical devices, usually implantable, that can be connected to or that are permanently connected to electrode leads for delivery of electrical stimulations pulses to the tissue (myocardium) of a human heart. Dual chamber pacemakers are capable of generating stimulation pulses for the atrium and the ventricle of a human heart. Biventricular pacemakers usually are capable to stimulate at least three chambers of a human heart that is the right atrium, the right ventricle and the left ventricle.
In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart.
Separate stimulation pulse generators are usually provided for each heart chamber (atrium or ventricle) to be stimulated.
A control unit is triggering the generation of a respective atrial or ventricular stimulation pulse according to a pre-programmed, variable timing regime in order to provide for adequate timing of the stimulation pulses.
A stimulation pulse to the myocardium may cause a contraction of a respective heart chamber, if the myocardium of that chamber is not in a refractory state and if the stimulation pulse has a strength above capture threshold of said myocardium. A sub-threshold stimulation pulse will not cause a cardiac contraction even if delivered to the myocardium in its non-refractory state.
Therefore, one important parameter to be adjusted in a pacemaker is capture threshold, which represents the minimum strength of a stimulation pulse required to reliably evoke cardiac contractions. This is typically determined by varying the strength (amplitude and/or duration) of applied stimulation pulses while simultaneously monitoring mechanical action or intracardiac electrical potentials produced during each contraction of the patient's heart. Capture is indicated when an applied stimulation pulse results in the occurrence of a heart contraction.
In order to determine capture threshold automatically, a heart stimulator can provide a cardiac capture threshold determination system for automatically determining the minimum pacing pulse energy required to reliably stimulate contractions of a patient's heart. The pacemaker's stimulation pulse generator is adapted to generate stimulation pulses of variable pulse strength for application to the heart, and a capture detector is provided for detecting cardiac contractions stimulated in response to the applied pacing pulses. The control unit is coupled to the stimulation pulse generator and responsive to the capture detector as to vary the pulse strength such that the pulse strength increases when the capture detector fails to detect cardiac contractions and decreases when the capture detector detects cardiac contractions.
In order to monitor the heart chamber and thus to determine whether or not a contraction of a heart chamber has occurred a pacemaker has a sensing stage for sensing a heart parameter indicating a contraction of a stimulated heart chamber.
The sensing stage can be connected to an electrode placed in a respective heart chamber. A contraction of a heart chamber can be detected by evaluating electrical potentials sensed by such sensing electrode. The time course of these potentials forms an intracardiac electrocardiogram that can be evaluated by the capture detector.
Alternatively, the sensing stage can be designed to response to the mechanical action of the heart. One way of detecting mechanical action of the heart is to evaluate an time course of intracardiac impedance.
Various alternative approaches for determining capture of a heart chamber and to discriminate an evoked response from a polarization artifact are known in the art.
Likewise, numerous approaches for providing cardiac pacemakers featuring automatic capture determination are known. One approach is disclosed in U.S. Pat. No. 4,708,142 directed to an Automatic cardiac capture threshold determination system and method
However, not much effort has been put into optimization of threshold search with respect to the regime of pulse strength alteration in an iterative search. Known approaches feature stepwise reduction or increase of pulse strength until loss of capture or capture is detected, respectively.
It is an object of the present invention to provide a heart stimulator with improved automatic capture threshold adaptation.
It is a further object of the invention to provide a heart stimulator with improved capture control.